The present invention relates to an optical measurement technology, and particularly to an optical measurement method and instrument for receiving optical signals from a living body to obtain information on the living body. More particularly, the present invention relates to removing noise signals superimposed on the information on the living body to highly precisely obtain desired living body signals.
Besides targeted signals, there are also noise signals, which are caused by several factors, included in measurement signals that are obtained by measuring information inside the living body using light (e.g., Patent Document 1). An averaging method, a band-pass filter, and the like are used to reduce the effects from the noise signals.
A technique using the averaging method reduces the noise signals based on a precondition that a brain activity responds to the same assignment in the same manner.
A technique using the band-pass filter reduces the noise signals based on a precondition that a brain activity signal and a noise signal exist in different frequency bands. These techniques process a plurality of noise signals. However, a technique is also proposed that is specialized in processing noise signals caused by special factors.
For example, noise signals caused by pulsations (pulsation noise signals) can be reduced by means of a band-cut filter, because their frequency is readily identified. A technique is proposed that reduces pulsation noise signals included in optical measurement signals based on the pulsation signals measured at an area, such as an ear (Patent Document 2). A technique is also proposed that reduces effects from the pulsation noise signals by extracting the pulsation noise signals from the optical measurement signals themselves (Non-Patent Document 1).
Moreover, a technique is also proposed that divides optical measurement signals measured at many points into a plurality of signals by a signal analysis method, and extracts noise signals and target signals (Patent document 3).    [Patent Document 1] JP-A-09-135825    [Patent Document 2] JP-A-2004-173751    [Patent Document 3] JP-A-2005-143609    [Non-Patent Document 1] Maria Angela Franceschini et al., NeuroImage 21 (2004) 372-386    [Non-Patent Document 2] Sato et al., NeuroImage 21 (2004) 1554-1562    [Non-Patent Document 3] Hirosaka et al., Physica D 194 (2004), Pages 320-332